The objective of the proposed research is to investigate how photoreceptors encode information about decreases in light intensity under light adapted conditions. The long range goal of this research is to arrive at a clearer understanding of the transduction of visual information into neuronal signals. The research plan is divided into two parts. Research will be continued on the distal hyperpolarizing photoreceptors of the mollusc Pecten irradians. These receptors are designed to respond to decreases in light intensity under light adapted conditions (off receptors) and therefore may provide a convenient biological model for aspects of vertebrate cone function. Our results indicate that the off depolarization of these cells produced by diminution or cessation of light intensity is spectrally dependent and involves photoproduct accumulation. The spectral dependence of the early and late receptor potentials will be investigated. Our results also indicate that the off depolarization involves more than one change in membrane conductance. The ionic basis for these changes in conductance will be investigated with current injection and ionic substitution experiments. Research will be started in the off response (off depolarization) of vertebrate cones in the turtle retina to a decrease in illumination under light adapted conditions. An optical stimulator has been designed to elicit the off response with dark pulses of variable intensity. The optimal conditions for eliciting this response will be determined. Experiments involving current injection and ionic substitution are designed to determine the ionic mechanism(s) involved in the off response. The possible dependence of the cone off response on the spectral composition of the stimulus will be examined.